This looks like a pretty solid study: equip people with accelerometers and tell them to walk for 30, 60, or 90 minutes per day. What they found:
A 30-min prescription of extra walking 5 times per week was well tolerated. However, in order to increase total PA further, much more than 60 min of walking may need to be prescribed in the majority of individuals. While total exercise ‘volume’ increased with prescriptions longer than 30 min, compliance to the prescription decreased and greater compensation was evident.
This isn’t an especially shocking conclusion. I consider long-term compliance more important to a physical fitness routine than its efficiency or its effectiveness—if it’s so difficult that you give up after a few weeks, it’s not effective. “Difficult” doesn’t even have to be the defining negative characteristic; it could be too inconvenient, too time-consuming, too expensive, etc.
A lot of people, in their attempts to go out and “be fit”, go too hard and too heavy too early on. They run until they’re tired every single day, get burned out, and give up. As with anything new, it is more important to low-ball one’s initial efforts, allow time to adapt, and think in terms like “gradual” and “long-term.” Don’t set yourself up for failure.